The present invention relates to a process for production of stretched moldings. More particularly, it is concerned with a process for producing stretched moldings which are equivalent or superior to moldings of super or ultra high molecular weight polyethylene in mechanical properties and which are excellent in properties such as adhesive properties, dyeability and print-ability.
Ultra high molecular weight polyethylene is excellent in properties such as impact resistance, abrasion resistance and self-lubricating properties, and its mechanical strength and modulus of elasticity can be increased by highly stretching. Thus it is increasingly finding applications as an engineering plastics.
This super high molecular weight polyethylene, however, has disadvantages of being poor in adhesive properties and dyeability because of its chemical stability, and thus is limited in its use. In particular, poor adhesive properties place a serious hindrance in production of composite material using ultra high molecular weight polyethylene. For example, when ultra high molecular weight polyethylene is used as a reinforcing material in production of e.g., concrete, helmets or pressure containers, adhesive properties to the matrix or other materials are poor.
Various surface treatments are applied to improve adhesive properties and dyeability of ultra high molecular weight polyethylene. For example, a sanding method, a flaming method (heat treatment) or an oxidizing method is applied. These methods, however, deteriorate the performance inherent to the polymer because they cause molecular cutting or formation of structural defects. Another method is to graft polymerize a polar group-containing monomer. This method, however, causes branching of the molecule or produces a domain. Thus in accordance with this method, it is difficult to produce moldings having excellent performance. Moreover, because these methods are post treatment, one additional step is needed, and problems arise in difficulties in determining conditions and in production costs.
A number of methods have been proposed for production of ultra high molecular weight polyethylene, but a method for production of ultra high molecular weight ethylene-based copolymer has not been disclosed. The reason for this is that such ultra high molecular weight ethylene-based copolymers are limited in their use, because chain transfer to the comonomer frequently occurs and their mechanical properties are much inferior to those of ultra high molecular weight polyethylene.
An ethylene-(meta)acrylate copolymer has heretofore been produced by the high pressure radical polymerization method (200.degree. C. and more than 1,000 atmospheric pressure).
In this method, however, in the presence of a catalyst chain transfer involves monomer, solvent and so on, including chain transfer between molecules and, therefore, a weight average molecular weight (Mw) of more than 1,000,000 which is commonly called a ultra high molecular weight is not attained; usually the weight average molecular weight is only less than 100,000.
The copolymer obtained by the above conventional method is branched in structure. Thus even if a copolymer having a super high molecular weight can be obtained, features characteristic of ultra high molecular weight polymer, such as high stiffness, are not sufficiently exhibited.
In Japanese Patent Publication No. 23317/1974 and Japanese Patent Application Laid-Open No. 278508/1986, for example, a method of copolymerizing ethylene and unsaturated carboxylic acid or its ester under low pressure in the presence of Lewis acid is disclosed. In accordance with this method, however, a so-called ultra high molecular weight ethylene-based copolymer has not obtained.
As copolymers of ethylene and polar group-containing monomers (e.g., an ethylene-ethyl acrylate copolymer), as described above, copolymers having a high ethylene content have been obtained by the high pressure radical polymerization method. In these copolymers, however, the molecular weight is low, and long chain branches are present. Thus a copolymer excellent in mechanical strength, modulus of elasticity and so on like ultra high molecular weight polyethylene has not been obtained.